1. Field of the Invention
This invention relates to a thermal spraying composite material containing molybdenum boride, and more particularly, to a thermal spraying composite material for forming a coat to protect mechanical equipment from corrosion by any molten light metal or alloy, such as aluminum, zinc or alloys of these.
2. Description of Related Art
Die casting, gravity casting, or differential pressure casting have been usual processes for casting a product from a metal having a relatively low melting point, such as aluminum, zinc or magnesium.
Differential pressure casting is, among these, considered suitable for making large castings having fewer internal defects. FIG. 2 shows an apparatus employed for differential pressure casting. A suction port 6 is used to create a lower pressure in a mold 5 than in a holding furnace 1, so that a molten metal 10 may rise from the holding furnace 1 through a stoke 2 and form a laminar flow through a sleeve 4 to fill the mold 5 for one cycle of the casting operation. When the molten metal has solidified on the inner surface of the mold 5, the next cycle of the casting operation is started, and the remaining molten metal flows down into the holding furnace 1 through the sleeve 4.
The sleeve 4 has its inner surface washed by the molten metal 10 having a high temperature during each cycle of the casting operation, and thereby corroded, and is eventually fractured. The higher temperature the molten metal has, the shorter life the sleeve 4 has.
The molten light alloys have usually been used for casting at relatively low temperatures in the range of 700-750xc2x0 C. and the protective coats of the sleeve 4 and the mold, have been made of, for example, a mixture of tungsten carbide and cobalt having a cobalt content of 12% by weight, as described in the Japanese Unexamined Patent Application No. Hei 7-62516.
Many foundries have, however, come to employ higher molten metal temperatures in the range of 750-850xc2x0 C. for making products of higher accuracy by the differential pressure casting.
When exposed to any such higher molten metal temperature, the protective coats of tungsten carbide and cobalt have been found lacking in durability, and particularly in oxidation resistance, and heavily worn by oxidation not only on the sleeve 4, but on the inner surface of the mold 1 as well, owing to low oxidation resistance of tungsten carbide at high temperature. A greatly shortened mold life has led to increase in the cost of the casting operation.
Under these circumstances, it is an object of this invention to provide a thermal spraying composite material which can form a protective coat having an improved durability when exposed to a molten light alloy having higher temperatures.
This object is attained by a thermal spraying composite material comprising:
molybdenum boride (MoB), from about 30 to about 70% by weight;
nickel (Ni) or cobalt (Co), from about 20 to about 40% by weight;
chromium (Cr), from about 5 to about 20% by weight; and
at least one metal boride selected from the borides of Cr, W, Zr, Ni and Nb, from about 5 to about 10% by weight.
It is another object of this invention to provide a thermally sprayed coat having improved durability when exposed to a molten light alloy having higher temperatures.
This object can be achieved by a coat comprising:
a first layer formed on a substrate to be protected from a heat-resisting alloy having a coefficient of thermal expansion close to that of the base;
a second layer formed on the first layer from a material comprising from about 30 to about 70% by weight of molybdenum boride (MoB), from about 20 to about 40% by weight of nickel (Ni) or cobalt (Co), from about 5 to about 20% by weight of chromium (Cr), and from about 5 to about 10% by weight of at least one metal boride selected from the borides of Cr, W, Zr, Ni and Nb; and
a third layer formed on the second layer from a ceramic material with low wettability to any molten light metal.
The first layer serves as a buffer between the substrate to be protected and the second layer of a composite material containing molybdenum boride, and is preferably of an alloy having a coefficient of thermal expansion between those of the substrate and the second layer. It may alternatively be formed by thermally spraying a metal having a coefficient of thermal expansion close to that of the second layer, and a good compatibility with the base.
The second layer plays the most important role in protecting the substrate from corrosion by any molten light alloy having a higher temperature. The role will be described in further detail.
The third layer is a very hard layer serving to protect the second layer from any physical damage otherwise given to the second layer by a violently flowing molten metal, or any other external force, as produced by striking.